There are a
number of wafer cleaning techniques or steps employed to ensure that a
semiconductor wafer is always free of contaminants and foreign materials
as it undergoes the wafer fabrication process.
Different contaminants have different properties, and therefore have
different requirements for removal from the wafer. Below are some
examples of commonly-used methods for wafer cleaning.
or simply 'resist stripping', is the removal of unwanted photoresist
layers from the wafer. Its objective is to eliminate the
photoresist material from the wafer as quickly as possible, without
allowing any surface materials under the resist to get attacked by the
chemicals used. Resist stripping can be classified into: 1)
organic stripping; 2) inorganic stripping; and 3) dry stripping.
organic strippers, which are chemicals that break down the structure of
the resist layer. The most widely-used commercially available organic
strippers used to be the phenol-based ones, but their short pot life and
difficulties with phenol disposal made low-phenol or phenol-free organic
strippers the more popular choice nowadays.
strippers, which are also known as oxidizing-type strippers, are used
usually to remove photoresist from non-metallized wafers, as well as
post-baked and other hard-to-remove resists. Inorganic strippers
are solutions of sulfuric acid and an oxidant (such as ammonium
persulfate), heated to about 125 deg C.
pertains to the removal of photoresist by dry etching using plasma
etching equipment. Its advantages over wet etching with organic or
inorganic strippers include better safety, absence of metal ion
contamination, decreased pollution issues, and less tendency to attach
underlying substrate layers.
Removal of Film Contaminants
bonded films of contaminant material may be removed from a wafer surface
cleaning comes in various forms, depending on the nature of the film
contaminants that need to be removed and from which surface. In general,
however, chemical cleaning is performed with a series of
As an example, removal of film contaminants from a wafer with nothing
but thermally grown oxide may consist of the following steps:
preliminary cleaning, removal of residual organic contaminants and some
metals, stripping of the hydrous oxide film created by the previous
step, desorption of atomic and ionic contaminants, and drying.
Storage of cleaned wafers must be avoided but, if necessary, must be
done using closed glass containers inside a nitrogen dry box.
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A thin oxide
layer grows over silicon or aluminum when these are exposed to air,
oxide layers need to be removed in places where they shouldn't be,
because they exhibit adverse effects such as higher contact resistance
or hampered interfacial reactions.
are the techniques commonly utilized to remove these native oxides from
the wafer. "In-situ" means performing the sputter or plasma
etching in the same vacuum environment where the overlying layer will be
contamination of wafers with insoluble particulates is also a common
problem. There are two frequently utilized techniques for removing
particulates from a wafer: 1) ultrasonic scrubbing; and 2) a combination
of mechanical scrubbing and high-pressure spraying.
consists of immersing the wafer in a liquid medium which is supplied by
ultrasonic energy. The sonic agitation causes microscopic bubbles
to form and collapse, creating shock waves that loosen and displace
particles. Ultrasonic scrubbing requires a filtration system that
removes the particles from the bath as they get detached. One drawback
of ultrasonic scrubbing is that they can cause mechanical damage to
employs a brush that rotates and hydroplanes over a solvent applied on
the wafer surface. This means that the brush does not actually
contact the wafer, but the solvent moved by the rotating brush dislodges
particles from the wafer surface. As this happens,
of a D/I water jet over the wafer surface is done to help in clearing
the wafer surface of particulate contamination.
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